The invention relates to a device for coating a substrate by means of plasma-chemical vapour deposition or cathode sputtering, said device having an evacuated chamber, a first and a second electrode, means for introducing gaseous substances into the chamber, and means for applying an RF-voltage to the first and/or the second electrode to produce a plasma.
Such a device may be used, for example, for coating a substrate with a layer of polymerized material by a glow discharge polymerization or also as a device for a cathode sputtering process.
Glow discharge polymerization is a known technique for depositing a layer of an organic or inorganic polymer. There are two fundamental types of methods.
In the first type the surface of an existing material is polymerized and cured by being exposed to a glow discharge which is produced in air or in an inert gas. Surface molecules are activated by the glow discharge and form compounds and cross-linked compounds with adjacent molecules. Since the activation is limited to an area in the proximity of the surface, the mass of the material remains unchanged. In the second type of method a different layer of polymerized material is deposited on a substrate by a glow discharge that is produced in a monomeric gas adjoining the substrate. Reactive species and types of material, respectively, which are produced in the glow discharge are deposited on the substrate and form a polymerized layer. The polymerization extends through the whole deposited material. The present device relates to the performance of polymerization methods of the second type.
In a typical polymerization situation of the second type the flow discharge is produced by means of an electric potential which is applied to two electrodes which are provided in a space which comprises a monomeric gas at a pressure less than atmospheric pressure. A glow discharge occurs and only a very small current flows through the gas, when the potential between the electrodes does not exceed a threshold value, which value is sufficient to produce ionization of the gas or disruptive discharge.
It is known that this discharge potential depends on the composition of the gas, the pressure of the system and the spacing between the electrodes. After the discharge has occurred, the gas is conductive and a stable plasma can be maintained over a wide current range. Once a plasma has been produced, it can be maintained by a potential which is smaller than the breakdown potential and the breakdown voltage, respectively. The accurate composition of the discharge plasma is not known. It is assumed that it consists of electrons, ions, free radicals and other reactive species.
Cathode sputtering is also a known method for forming a layer of material on a substrate.
In cathode sputtering the material is removed from the surface of a target plate by ion bombardment and is deposited on the substrate. When the material to be sputtered is electrically conductive, a direct voltage potential is used. When the material to be sputtered is not electrically conductive or is an insulator, it is preferred to use a high frequency voltage to avoid the formation of surface charges on the insulator and the loss of accelerating voltage resulting therefrom. In known methods of depositing a layer of a polymer by cathode sputtering, first the polymer itself is prepared in the form of a sheet, respectively, or a powder target. A target electrode is manufactured by arranging the target in contact with a conductive surface. The target electrode and the target, respectively, and a second electrode are provided in an evacuated space which may be filled with an ionizable inert gas, for example argon, at a suitable pressure. A polymeric layer can be formed on a substrate by a glow discharge which is produced by applying a suitable voltage between the target and the second electrode.